Shall be in you - future tense. I have stated one can have faith in Christ from the OUTWARD witness of the Spirit. THEN by/through that faith receive Christ and become born of God. Those in the OT were not born again. The vail in the holy of holies was torn in two at Christs last breath.
God spoke of that inward cirumcision or writing of a new covenant on the hearts of the people . Jesus introduced that covenant in His blood.
What does the outward witness of the Spirit even mean? Does it mean signs, wonders and miracles? If so, then no, it would be impossible to garner true faith from that. Were it possible, then the Jews of the exodus who had observed first-hand God's power, might, and miracles, to include the hearing of God's own voice by the mount that burned, and it being a sight so terrible that even Moses said that he exceedingly feared and quaked, the Jews should have unconditionally, permanently, followed after God. Therefore, based upon your theory of OT faith if true, and seeing and hearing what they did, they should never have never strayed unto false gods, nor to not worship the one true God. But nevertheless, stray they did. So, your theory (if I've described it correctly), cannot possibly be right. Demonstrations of any kind, even those directly from/by God, but being without the Holy Spirit present, could not/did not bring them to faith. Faith isn't an intellectual process it is a spiritual gift. To have faith, one must have Christ. To receive Christ is from the Holy Spirit. Christ Himself is the sole author and finisher of faith.
[Deu 4:11-13 KJV]
11 And ye came near and stood under the mountain;
and the mountain burned with fire unto the midst of heaven, with darkness, clouds, and thick darkness.
12
And the LORD spake unto you out of the midst of the fire: ye heard the voice of the words, but saw no similitude; only [ye heard] a voice.
13
And he declared unto you his covenant, which he commanded you to perform, [even] ten commandments; and he wrote them upon two tables of stone.
[Heb 12:20-21 KJV]
20 (F
or they could not endure that which was commanded, And if so much as a beast touch the mountain, it shall be stoned, or thrust through with a dart:
21 And so terrible was the sight, [that]
Moses said, I exceedingly fear and quake
Didn't you read the verses I posted or are you ignoring them? They clearly state that the Holy Spirit was with those saved during the OT. If the Spirit didn't change their hearts and minds, then no outward demonstrations would do so and would be meaningless to them. Only the presence of the Spirit being with an individual had made that happen, man, either in OT or NT, of himself, could not achieve it.
The Holy Spirit was in David DURING the OT.
[Psa 51:11 KJV] 11 Cast me not away from thy presence; and
take not thy holy spirit from me.
The Holy Spirit was in Abraham DURING the OT
[Gen 41:38 KJV] 38 And Pharaoh said unto his servants, Can we find [such a one] as this [is],
a man in whom the Spirit of God [is]?